1. Field of the Invention
The present invention relates to a piezoelectric device, a liquid discharge head that employs this device and a liquid discharge apparatus.
2. Description of the Related Art
A piezoelectric device is a functional device that utilizes a piezoelectric phenomenon, i.e., a piezoelectric effect whereby an electric field is generated upon the application of a stress to a piezoelectric material, or a converse piezoelectric effect whereby a stress is produced by applying an electric field to a piezoelectric material.
A device that utilizes the piezoelectric effect is employed, for example, as a piezo spark ignition device, an acceleration sensor device, a gyro angular velocity sensor device or a strain gauge device. A device that utilizes the converse piezoelectric effect is employed, for example, as an ultrasonic vibration device, or a precision drive device for a loudspeaker or an SPM (Scanning Probe Microscope) stage. Further, recently, a device utilizing the converse piezoelectric effect has been employed, for example, as a discharge drive device for an ink jet head and as a drive device for an MEMS.
Piezoelectric devices that include a function for employing the converse piezoelectric effect to perform fine, precision driving can be roughly classified as belonging to two categories, in accordance with a direction, relative to the direction in which an electric field is applied, in which strain and stress, generated by a piezoelectric device, due to the converse piezoelectric effect are to be obtained.
The first category is a d33 mode type or a vertical vibration type that employs a strain/stress (horizontal effects) in the direction (the direction d33) parallel to the direction to which an electric field is applied. The other category is a d31 mode type or a transverse vibration type that employs strain/stress (vertical effects) in a direction (the direction d31) corresponding to the vertical plane in the direction in which an electric field is applied. Further, in a case wherein strain/stress in a shearing direction (the direction d15) is employed relative to the direction in which electric field is applied, a piezoelectric device can be categorized as being a transverse vibration mode type or a vertical vibration mode type, mainly in accordance with the direction in which strain is obtained.
When with the vertical vibration mode, especially, strain/stress in the direction d33, is employed for a piezoelectric actuator, a displacement that is caused by stress/strain parallel to the input electric direction of a piezoelectric material is employed relative to the direction in which the electric field is input. Because of the property of the piezoelectric material, the amount of displacement at this time is only about 1%, at most, of the thickness of the piezoelectric material to which the input electric field is applied. Therefore, in order to obtain a large displacement, the piezoelectric material to which an electric field is to be applied should be thick. However, when the thickness is increased, an electric field must be applied at an intensity consonant with the thickness in order to obtain a displacement that matches the performance of the piezoelectric material, and therefore, the drive voltage must be raised. Thus, in order to resolve this problem, a configuration has been proposed whereby, in order to prevent an increase in the drive voltage, electrodes are laminated so that a drive electric field is applied to a piezoelectric member in a direction perpendicular to a displacement extraction direction.
When in the transverse vibration mode, especially, strain/stress in the direction d31, is employed for a piezoelectric actuator, the stress/strain exerted on the plane perpendicular to the direction of the input electric field for a piezoelectric member is employed relative to the input electric field direction. At this time, generally, the displacement of the piezoelectric member is much smaller than in the vertical vibration mode. However, in the mode d31, when the piezoelectric device is bound to the substrate, the substrate is bent as the piezoelectric member is distorted during the transverse vibration mode, and as a result, a greater displacement can be obtained, even though the thickness, in the direction in which the electric field is applied, is small. Additionally, since the thickness in which the electric field is applied is small, a low drive voltage can be set, so that a piezoelectric actuator of a bending mode type is generally employed.
The types described above are appropriately selected in accordance with the use of a piezoelectric actuator.
However, for a precision drive source for an MEMS that includes a fine structure, or a high-density multi-nozzle ink jet head that is applied for an ink jet apparatus for which high speed and high image quality are required, a piezoelectric actuator having the following configuration is effective. This piezoelectric actuator, of a bending mode type, employs the transverse vibration mode d31 employed by a piezoelectric member. Through precise processing, the piezoelectric member can be deposited directly on a substrate that includes drive power supply electrodes, using a thin film deposition technology.
However, since the piezoelectric member deposited by the thin film deposition technology is deposited in a state bounded to the substrate that includes the drive power supply electrodes, the piezoelectric property is not satisfactory, when compared with that of a piezoelectric member made of bulk ceramics.
As a relation between piezoelectric constants d33 and d31, for example, for bulk ceramics, individual components d33 and d31 of PZT (Pb (Zr, Ti)O3), for which a Zr/Ti ratio is allocated, are described in Ceramic Dielectrics Engineering, page 334 (Kiyoshi Okazaki, Gakuken Co., Ltd.) (Non-patent Document 1). A graph obtained by plotting the values of the constants d33 and d31 is shown in FIG. 2. The line running from the origin of the graph in FIG. 2 is a line obtained by linear approximation based on plotted data, and inclination d33/|d31|=2.5 is represented.
In Japanese Patent No. 3256254, a piezoelectric material is proposed as a material appropriate for an underwater transducer (hydrophone) that utilizes inverse piezoelectric effects and piezoelectric effects. This material is a composite material, composed of organic and inorganic materials, such that by defining the piezoelectric constant d31 as substantially zero, a large piezoelectric constant dh under a hydrostatic pressure, dh=d33+2×d31, can be obtained.
The piezoelectric constant d31 can be determined by using a unimolph cantilever method. A relation of the amount of displacement of a unimolph cantilever relative to an input voltage V is written in J. G. Smith and W. Choi, “The Constituent Equations of Piezoelectric Heterogeneous Bimorph”, IEEE trans. Ultrason. Ferro. Freq. Control 38 (1991) 256-270. (Non-patent Document 2). The material property values of the relations are written in W. Cao, “Full Set Material Properties of Multi-Domain and Single-domain (1-x)Pb(Mgl/3Nb2/3)O3-xPbTiO3 and (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 Single Crystals and the Principle of Domain Engineering Method”, Piezoelectric Single Crystals and Their Application (pp. 236-256), edited by S. Trolier-Mckinstry, L. E. Cross and Y. Yamashita (Non-patent Document 3).